Conventionally processed helical coil springs possess a residual stress distribution that is not ideal for durability. Using known spring-coiling processes, the highly-stressed inner diameter of the resulting spring is placed in a state of residual tensile stress after coiling. Even after inducing a layer of residual compressive stress via shot-peening or otherwise, there exists a sub-surface state of residual tensile stress. This residual tensile stress is undesired and leads to excess fatigue and premature spring failure. As such, it is highly desirable to eliminate the residual tensile stress and/or to completely reverse same by imparting residual compressive stress to the spring during its manufacture or by subsequent treatment.
Back-bending by forced-arbor coiling is one example of a process used during coiling of the spring to alleviate residual tensile stress at the spring inner diameter. Post-coiling techniques for residual stress enhancement include shot-peening, piece hardening and nitriding. All of these known techniques are associated with undesired consequences such as reduced hardness, increased variation/distortion, extreme brittleness and/or greater risk of introducing defects.
The above residual stress enhancement techniques do not yield springs having sufficiently large residual compressive stress, i.e., −40 ksi (1 ksi=1000 lb/in2) and below, at extended depths, i.e., deeper than 0.008″, moving into the wire from which the spring is formed from the inner diameter of the spring toward the outer diameter of the spring.
With the advent of improved wire surface quality as well as improved spring manufacturing techniques, one of the most common failure modes of engine valve springs is high cycle fatigue due to the inevitable impurities in the steel. These non-metallic inclusions commonly initiate fatigue cracks after a significant number of cycles, and at a depth below the surface where compressive stress from shot-peening is either low or non-existent.